1. Field of the Invention
The present invention relates to a differential amplifier, particularly to a variable amplitude type differential amplifier capable of producing an output having an arbitrary magnitude of variation, i.e., amplitude.
2. Prior Art
A circuit configuration of a prior art variable amplitude type differential amplifier is illustrated in FIG. 2.
The differential amplifier in FIG. 2 produces an output 11, and has differential signal sources 9 and 10, transistors 1 to 4, resistors 5 and 6, a voltage source 7 and a constant current source 8. The constant current source 8 includes a voltage source 12, a control voltage source 13 and a transistor 8a. The transistors 1 and 3 are connected in parallel with each other, and the transistors 2 and 4 are also connected in parallel with each other, whereby the differential amplifier includes the transistors 1 to 4 having common by connected emitters.
In the prior art circuit configuration in FIG. 2, the currents flowing into the resistors 5 and 6 are varied by applying input signals from the differential signal sources 9 and 10 to the bases of the transistors 1, 3 and 2, 4 respectively, so as to vary the control voltage source 13 of the constant current source 8. An arbitrary range of amplitude is produced at the output 11 due to the voltage drop caused by the current flowing from a source voltage which is rated by the voltage source 7 into the resistor 5.
Since the transistors 1, 3 and 2, 4 are connected in parallel with one another, the current flowing into the resistor 5 is divided to flow into the transistors 1 and 3, while the current flowing into the resistor 6 is divided to flow into the transistors 2 and 4. That is, since the transistors 1, 3 and 2, 4 are connected in parallel with one another, the amount of current flow per transistor is reduced.
However, in the prior art differential amplifier in FIG. 2, for example, if the amplitude 0.1 V is intended to be produced at the output 11 in case that the resistor 5 has a resistance value 50.OMEGA., the current flowing into the resistor 5 must have a value 2 mA. Since the transistors 1, 3 and 2, 4 are connected in parallel with one another, the amount of current to flow per transistor is further reduced to have a value of 1 mA. If the amplitude 5 V is intended to be produced at the output 11, the current flowing into the resistor 5 has a value of 100 mA, and the amount of current to flow per transistor 1, 3 has a value of 50 mA.
In the prior art differential amplifier in FIG. 2, if an output having large amplitude is intended to be produced at the output 11, it can not be produced at the output 11 unless the large amount of current flows into the resistors 5 and 6 in case that the resistant values of the resistors 5 and 6 are small. Considering the maximum rated current flowing into the transistors I to 4, the number of transistors to be connected in parallel with one another must be increased in order to reduce the current to flow per transistor. As a result, parts necessary for the differential amplifier are increased in number, and the power consumption is increased since a large amount of current flows into the resistors 5 and 6.
If the resistance values of the resistors 5 and 6 are made large, the current flowing into the resistors 5 and 6 can be reduced. However, in this state, if an output having the small amplitude is desired at the output 11, the amount of current flowing into the resistors 5 and 6 is reduced. Further, since the transistors 1, 3 and 2, 4 are connected in parallel with one another, the amount of current to flow per transistor becomes very small. Accordingly, in view of the I.sub.C -f.sub.T characteristic of the transistor, when the amount of collector current flowing into the transistor is very small, the frequency characteristic of the transistor is inferior, so that the frequency characteristic of the variable amplitude type differential amplifier is then deteriorated.
As mentioned above, there is a problem in the prior art variable amplitude type differential amplifier wherein the output amplitude can not be varied to a large extent with low power.